For industrial purposes, it is frequently necessary to rapidly combine streams of liquids and solids to form solutions on a continuous basis. The problems encountered in forming uniform solutions by mixing powdered or granulated solids with liquids have been researched extensively. However, no suitable means for dissolving hard-to-wet materials, such as certain polymers, has previously been discovered to prepare solutions of 5-95% dissolved solids.
Many water soluble polymers, such as polyvinyl alcohol, hydroxyethyl cellulose, carboxymethyl cellulose and the like, are extremely soluble in water but are nevertheless very difficult to dissolve. The polymer particles adhere strongly to one another on wetting and tend to form lumps. In most traditional mixing devices, such lumps become wetted before the particles disperse into individual particles. The wetted surface of a lump becomes an impermeable film that hinders break up of the lump, and the lumps are carried through the mixer with the powder inside remaining substantially dry and unmixed with the liquid.
Another problem encountered in forming solutions of polymer powders is the release of substantial amounts of dust. Depending on the type of material, airborne dust may present health hazards. For example, polymer dust is extremely slippery when wet and thus may present a safety hazard to workers if it forms on surfaces around the mixing device. It may also be a respiratory irritant. The dust may also present a process problem if it accumulates on the interior surfaces of the mixing equipment. If the surfaces are moist but not washed, a wet paste can build up to plug passages of the equipment.
A traditional solution to the problems of preparing solutions of hard-to-wet polymer powders is a batch process. In an example of a traditional method, ambient temperature water is charged to a blend tank, and the water is agitated to form a vortex. The powder is then dispersed in the ambient water by gradually adding it to the vortex. The agitated mixture of powder and water is heated using, for example, steam sparging or jacket heating to a specific cure temperature. The mixture is held and agitated at the cure temperature for the time required to dissolve the powder. Polyvinyl alcohol, for example, is first formed into a slurry in ambient temperature water and then heated to a temperature of at least 90.degree. C. Under these conditions, the complete dissolution of the slurry typically takes 30 to 60 minutes and yields no greater than a 10% solution. Hydroxyethyl cellulose is another hard-to-wet powder which, although curing at ambient temperature, requires at least two hours to form a complete solution.
There are many disadvantages with the traditional method. A fundamental problem is that the method is inefficient, costly, capital intensive and time-consuming. The powder is added to ambient temperature water with high agitation to disperse the powder. If the water is at an elevated temperature, the powder clumps more readily. Once the powder is relatively well dispersed, the mixture must be heated and held at the higher temperature in order to dissolve the polymer. The mixing, heating, and curing cycle is slow. In addition, the space required for the blend tank may present a problem in installing a polymer solution system in an existing plant. Also problematic is that undissolved powder clumps can remain in the solution and result in inconsistent solution properties. Solution aeration due to the high speed agitation required for polymer dispersion and excessive foaming due to the heat-curing requirement are additional problems.